By x-ray irradiation of a mixed KCl-KF crystal at liquid-nitrogen temperature, a molecule ion species identified as F${\mathrm{Cl}}^{-}$ can be formed oriented along a $〈111〉$ axes of the crystal. The electron structure of this electron-deficient center has been investigated by electron spin resonance. The parameters of the appropriate spin Hamiltonian have been determined and are compared with those for ${\mathrm{F}}_2^{-}$ and ${\mathrm{Cl}}_2^{-}$. A tentative assignment of signs for the isotropic parameter $a$ is suggested.

• Received 11 July 1963

DOI:https://doi.org/10.1103/PhysRev.132.1950